Cleaning Historic
Stained Glass Windows

Léonie Seliger

Mould growth

Stained glass and leaded windows
in churches do not normally require
regular cleaning. There are, however,
reasons why cleaning may become necessary.
This article provides an introduction to
the types of soiling found on the internal
and external faces of stained glass,
when and how to clean them safely and
sympathetically, and the kinds of damage
that can result from inappropriate cleaning.

SURFACE DEPOSITS

Surface deposits and accretions on windows
come in a great variety of forms, on both
internal and external surfaces.

On the external surface
Rainwater running down the outside of the
building and onto the windows slowly deposits
particles of the surrounding materials onto the
glass surfaces. These deposits include limescale
from render, mortar and limestone; and rust
from ironwork. Over time they form a thin but
very tough patina. On stained glass windows
this usually just mellows the intensity of the
sunlight passing through the glass but on clear
windows a dense patina can be quite intrusive.

Airborne particles can attach themselves
to the glass surface and to the leads. Heavy
traffic or industrial pollution can deposit thick
crusts, which are most visible in those areas
that are protected from direct rain, such as
at the top of a lancet, in small tracery panels
or under a horizontal bar. These crusts can
be quite loose and flaky, but they can also be
extremely hard. Tree sap may regularly coat a
window in sticky droplets, which then allow dust to adhere to the glass. Over time, this can
result in similar crusts.

Bird guano is another frequent nuisance.
Because there are serious health risks
associated with bird guano, it should only be
removed by trained people who are aware of the
risks and use the appropriate safety equipment.

Organic growth such as algae and lichens
(top right) can also be found on the outside
of windows.

On the internal surface
Even if a window is not leaking, water in
the form of condensation will regularly run
down the inside surface and can create thick
limescale deposits (facing page). Soot from
decades of burning candles can gradually
cause window glass to darken.
Algae, fungi and moulds (facing page)
are more often found on the inside of a
window, as there they have a regular supply
of condensation water but are not washed
away by rain.

CORROSION OF MEDIEVAL GLASS

Lichens on the external surface of medieval stained glass

Limescale deposits

Medieval glass will often show corrosion
damage and weathering crusts on the inside
as well as the outside.

On medieval glass, corrosion processes
can result in ‘weathering crusts’. These can
cover entire pieces evenly with brownish or
whitish crusts but they also often erupt from
distinct pits, covering the surface in white
spots (above). These crusts are the result of
a chemical interaction between the glass
and the atmosphere under the influence of
water. Underneath the weathering crusts
the medieval glass surface is always severely
damaged and very fragile.

DAMAGE CAUSED BY DEPOSITS

Some glass types are more prone to
damage than others and medieval glass
is particularly vulnerable. Some deposits
can be harmful to the glass while others
are not. Patina and some hard crusts,
although they can be unsightly, are unlikely
to cause damage to the underlying glass.

Soft deposits that attract and hold water
on the surface, and particularly organic
growth, can actively damage the glass by
keeping it damp. Organic growth often has
acidic metabolic by-products and, over time, can trigger corrosion damage even on post-medieval
glass and paint.

Painted decoration is particularly at risk.
Glass paint is similar to a pottery glaze and
is fused to the glass surface in a kiln before
the window is assembled. Windows of all
ages can have problems with poorly fired,
damaged or decaying glass paint (above and
right), and it takes an expert to tell whether
or not the paint is stable.

HOW TO CLEAN STAINED GLASS

Assessing the condition of glass and painted
decoration and advising on a suitable
cleaning method should always be carried
out by an accredited conservator, even if the actual cleaning can in some cases be
carried out by non-professionals. Even plain
unpainted glass may be very old and can be
damaged by the wrong choice of cleaning
method or by unskilled hands.

Church windows consist of a mosaic of
small glass pieces which are held together
with lead profiles and weather-proofed with a
grout. They are typically inserted into stone
surrounds and strengthened against wind
pressure with horizontal metal bars that are
inserted into the surrounding stonework and
attached to the windows with wire.

The complicated construction of church
windows means that they have to be cleaned
by hand, piece by piece. Depending on the
nature of the deposits, cleaning can be
time consuming and potentially damaging,
and may need to be carried out by skilled
professionals. In some cases, gentle and
comparatively quick cleaning with cotton
wool and distilled water can remove simple
loose dirt and bring a window back to its
original splendour. In other cases, cleaning
may be impossible, or can only be done under
a microscope in a conservation studio.

Typical corrosion damage to the exterior face of a medieval stained glass window

Loose flaking paint on medieval glass

Deposits on windows can include bits of
grit from surrounding stonework or rust from
iron bars, which could be trapped in the other
dust deposits. These can easily act like an
abrasive powder and scratch the glass surface.
Limescale is also often present and this can
leave very unsightly smear marks that may only become visible once the window has
dried completely.

The safest and most effective way to
clean historic windows that have no painted decoration, or where the decoration is in
sound condition, is to roll (not rub) cotton
buds dampened with a little de-ionised water
over the glass surfaces. The slightly damp
cotton fibres collect the dirt from the surface
very effectively. Cotton buds of the required
size can be made quite easily using bamboo
skewers and raw cotton, which is available
from most pharmacies. This enables the
cotton buds to be rolled to suit the size of the
piece of glass to be cleaned.

On unpainted glass, once the cotton bud
has stopped collecting dirt, a final polish
can be carried out to help the glass regain its
brilliance. It is always inadvisable to polish
painted glass.

In exceptional circumstances, for
example where dense rust staining (previous
page, bottom right) has caused unsightly
discolorations on plain glazing, more abrasive
methods like bristle brushes and plastic pot
scourers may be used as a last resort, but only
after taking professional advice.

DAMAGE CAUSED BY CLEANING

Paint loss on 19th-century glass

Rust staining before and after cleaning

The surface of glass is surprisingly prone
to scratches. It is quite wrong to assume
that only diamonds are hard enough
to scratch glass: wire wool, glass-fibre
brushes, abrasive powders and metal
tools can cause serious damage. While
the damage may not be immediately
visible, very small scratches can cause
corrosion, particularly in medieval glass.

The removal of corrosion deposits on
medieval glass is controversial, because over-cleaning
can expose very fragile corroded
glass surfaces. Cleaning of corroded glass should only be attempted by appropriately
trained, skilled and experienced conservators,
and should be done in moderation. A
corroded window will never look as good as
new, nor should it. Time has left its mark and
that change is part of the window’s history.

Over-cleaning of windows that
have fragile paint can result in severe
and irreversible damage, such as the
loss of painted detail, and can leave a
window unreadable. Unlike paintings
on canvas or paper, it is very difficult
to touch-in lost glass paint.

DOS AND DON'TS

Cleaning can improve the legibility and help
the long-term survival of a window, but it
must be done carefully and correctly.

Do seek the advice of an expert before
deciding to clean a window. It is always
worth getting good advice at the start –
once damage is done it cannot be undone.
For churches, the local diocese can often
help by recommending an advisor who
specialises in stained glass. Icon, the Institute
of Conservation, has a searchable online
register of accredited conservators (www.conservationregister.com).

Do provide safe access. Church windows
tend to be tall and are often at great height.
Safe access is important; it’s not worth risking
injury or worse for a clean window.

Don’t use harsh abrasive pads or
household cleaners and never use acids or
wire wool. Even the removal of cobwebs
should only be done extremely carefully and
the duster should never touch a window that
contains painted stained glass.

Don’t attempt to clean medieval stained
glass if you are not a trained and experienced
stained glass conservator.

The congregation of a small medieval parish church in Kent was
delighted to learn that their church contained fine stained glass
windows by Henry Holiday (1839-1927). Holiday was a Pre-Raphaelite
painter, designer, sculptor and illustrator, who succeeded Edward
Burne-Jones as designer of stained glass windows at Powell’s Glass
Works in London in 1861 and set up his own studio in 1891. One of his
best windows can be seen in Westminster Abbey.

The three small windows in St Mary’s Church were so densely
covered with dirt and microbial growth that no-one had given them
a second glance in decades. Ivy had invaded the space between the
rusting safety grilles, covering the stained glass even further. The
church is situated in a deep and damp valley, and is surrounded by
trees that grow very close to the building. This created a sheltered and
stagnant micro-climate with very high humidity and low light levels.
All of the church’s windows therefore showed dense micro-organism
growth on the inside of the glass.

In 2005, a survey of the stained glass windows was commissioned. It
was then that the partially obscured Henry Holiday glass was identified.

Close inspection revealed that the stained glass was not only
covered with dirt deposits, microbial growth and ivy, but that
underneath it all the delicate paintwork was in very poor condition.

The long exposure to stagnant condensation and to acidic metabolic
products from the micro-organisms had resulted in patchy losses of the
decoration. Losses were particularly severe where water was pooling on
the glass above horizontal lead profiles. The thin washes and the very
fine applied skin tone were mottled with patchy losses where algal mats
and moulds had invaded the surfaces.

The severity of the damage, the fragility of the remaining decoration
and the importance of the windows led to the relatively unusual decision
to protect them with external protective glazing. This decision was not
taken lightly, as it constituted a change to the building’s historic fabric.
Protective glazing is more usually installed where corroded medieval
stained glass needs protection from the elements.

In St Mary’s, however, even with planned improvements to the
heating system and some thinning out of the trees around the building,
the levels of humidity were going to remain high enough to cause more
damage to the already badly affected glass.

The stained glass was therefore removed from the glazing grooves
and taken to the conservation studio. The first treatment was the
repeated application of a fine mist of an ethanol-water mixture to kill
any active micro-organisms. Careful cleaning with small sable brushes
was then carried out, mostly under the microscope to avoid dislodging
any of the unstable paint.

Loose paint flakes were consolidated with
minute droplets of acrylic resin. At times it was deemed too hazardous
to remove the dead micro-organisms from very flaky paint layers. In
those cases they were left in place and consolidated together.

The protective glazing was installed into the original glazing
groove and sealed in. A condensation tray was installed at the bottom
of the outer glazing, with a drainage pipe that would evacuate run-off
to the exterior.

After conservation, the now beautifully crisp and jewel-like
Henry Holiday windows were framed with narrow bronze profiles and
attached to the stonework about 50mm behind the outer glazing. Small
slots at the bottom and at the top of the frames allow air to circulate
around the stained glass, keeping it completely dry and free from
condensation.

The external appearance of these three windows has certainly
changed, as the protective glazing has a much more reflective surface
than the historic stained glass. The western end of the church is
surrounded by trees and is little visited, however, so this change was
deemed acceptable.

Seen from the inside of the building, the west wall looks just as it did
before, except that Henry Holiday’s figures of patriarchs and angels now
shine again, and will continue to do so for a very long time to come.

The Building Conservation Directory, 2013

Author

LÉONIE SELIGER MA ACR is director of The Cathedral Studios,
Canterbury Cathedral. The studio
offers stained glass conservation, restoration and design services
throughout the British Isles.